My Acer Aspire 7520G-402G32Mi laptop freezes sometimes, with several Linux distros, and therefore did I reapply thernal paste and clean the fan. This was to no avail so I decided to try to run a memory test. I believe I forgot to mention btw, that before I did the thermal grease reapplication, for a couple of times it took several repeated attempts to boot (after pressing the power button, it went on for a sec, off, on for a sec and after a few times it finally booted). This issue seems to have disappeared now though. Memtesy 86+ gives this weird message after a couple of minutes:

Memtest86+ did a successful run with only the second dimm installed, so now it is time to repeat with only the first installed. There was one thing though, once I rebooted and booted Mageia (in order to do a shutdown) the freezing happened again. However the dialog with the shutdown/reboot/etc. options still appeared when pressing the power button so it was not fully unresponsive. Only the touchpad and keyboard stopped responding.
So I will run Memtest86+ again (I don't see a point in trying the one by Microsoft) and run the test for more than 1 loop just in case. I am not sure what to do if the semi-freezing happens with the other dimm too though.

Sounds like a bad mobo (bad dimms/etc) / shoddy power delivery in the device. Also the ram sticks could be going bad/degrading. I would see if it runs with different ram (maybe a cheapo 512mb dimm), and if not, then this laptop needs to be retired.

Its not worth it to replace the mobo, and if it is the power circuitry/battery then its only a matter of time before it stops turning on. Do you get freezing when on battery or only when plugged in (or does it not make a difference)?

Test results for the promised test with both battery and main power:
Because Mageia nagged about the need for booting without KMS support and gave those stripes again after I hit [OK], I thought it would be wise to put a bootable CD with Aptosid (basicly Debian) into it and reboot. It was like power on - power off - power on and booting. I got into the desktop succesfully and decided to use Gparted to check the partition Mageia was installed on. Just after that check (which was successful) the laptop stopped responding again to the touchpad and keyboard. Hitting the power button still worked though, the laptop recognised it as a shutdown command and started a regular shutdown procedure. But when the moment was there to remove the CD and press [enter] to power down the system, it did not respond to the [enter] keystroke. So I had to do a hard shutdown again.

Im not sure if this is relivant or not. but in my experience, the build quality of some of the older Aspire series laptops are a load of bollocks.

one of the most common problems seem to be the GPU overheating which i think you are experiencing given the pic of that 'white screen' similar to what this guy had on his old Packard Bell

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It does that 'whitescreen' thing from 0:17-0:30 where he is forced to do a hard reset by holding down the power switch.

the problem is, Instead of the heatsink inside making direct contact with the CPU/GPU they have a 0.02mm thick thermal pads like you get on some graphic cards when you remove the stock cooler and long story short - these thermal pads dont do a very good job.

however there are these small copper shims available on ebay or at some specialist repair shops that you can buy and put it between the CPU/GPU which bridges/fills the gap and makes sure the HSF is making direct contact with the heatsource.

I used to have a problem with X1600 Mobility Radeon in an older laptop that used to over heat and cause the laptop to go into thermal shut down. because BenQ who made the laptop, used the same crappy thermal pads instead of making it so the HSF makes direct contact with the CPU/GPU. However I bought a copper shim for it and after that i never had to worry about it overheating or thermal shut downs again.

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please pleaseeee don't attempt the heatgun trick unless you know what you are doing
always wrap everything in 3 layers of heavy metal foil and don't keep the heat in one spot move it in small circles ...